[Bug java/30999] New: support for GCC4.0's fvisibility option in JNIEXPORT macro

["jan dot nijtmans at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

For a patch implementing this proposal, see the end of this Description.

This proposal has been submitted to SUN as well, for inclusion
in a future JDK. Below is the full text of the proposal to SUN
and the answer.

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---------------------------------------------------------------


dateCreated: Fri Apr 07 07:41:29 MDT 2006
type:        rfe
cust_name:   Jan Nijtmans
cust_email:  jan.nijtmans@gmail.com
jdcid:       nijtmans
status:      Waiting
category:    java
subcategory: native_interface
company:     LogicaCMG
release:     5.0
hardware:    x86
OSversion:   linux
priority:    4
synopsis:    support for GCC4.0's -fvisibility option in JNIEXPORT macro
description: A DESCRIPTION OF THE REQUEST :
Currently, the macro JNIEXPORT is defined in jni_md.h as empty:
#define JNIEXPORT
This means that on UNIX, marking a symbols as exportable has no effect.

However, GCC 4.x has a new option -fvisibility=hidden which allows all symbols
which are not marked specially to be hidden from a shared library. This feature
is similar to Windows, where only symbols marked with  __declspec(dllexport)
are exported from dll's.

Therefore I suggest to change this line to:

#if defined(__GNUC__) && __GNUC__ > 3
#define JNIEXPORT __attribute__ ((visibility("default")))
#else
#define JNIEXPORT
#endif

This allows code to be compiled with gcc 4.x to be compiled with
-fvisibility=hidden, while still all JNI functions are exported.


JUSTIFICATION :
This enhancements makes it possible to reduce the size of JNI libraries
significantly. This helps the start-up time a lot. For more information, see:
     http://gcc.gnu.org/wiki/Visibility
This page explains much better what is going on that I could do it here.

For gcc 3.x and earlier or any other compiler, this change has no effect.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
If a JNI library is compiled with GCC 4.x using the option -fvisibility=hidden,
I expect that still all JNI functions are exported.
ACTUAL -
When using -fvisibility=hidden with GCC 4.x, all JNI functions would be hidden,
so this option is not usable with JNI libraries.

CUSTOMER SUBMITTED WORKAROUND :
A workaround is placing the following in all JNI code:

#include <jni.h>
#if !defined(_WIN32) && defined(__GNUC__) && __GNUC__ > 3
#undef JNIEXPORT
#define JNIEXPORT __attribute__ ((visibility("default")))
#endif
workaround:
comments:    (company - LogicaCMG , email - jan.nijtmans@gmail.com)



Index: libjava/include/jni_md.h
===================================================================
--- libjava/include/jni_md.h    (revision 122411)
+++ libjava/include/jni_md.h    (working copy)
@@ -124,6 +124,19 @@

 #define JNICALL          __stdcall

+#else /* !( _WIN32 || __WIN32__ || WIN32) */
+
+#define JNIIMPORT
+#if defined(__GNUC__) && __GNUC__ > 3
+#define JNIEXPORT __attribute__ ((visibility("default")))
+#else
+#define JNIEXPORT
+#endif
+
+#define JNICALL
+
+#endif /* !( _WIN32 || __WIN32__ || WIN32) */
+
 /* These defines apply to symbols in libgcj */
 #ifdef __GCJ_DLL__
 # ifdef __GCJ_JNI_IMPL__
@@ -135,14 +148,4 @@
 # define _CLASSPATH_JNIIMPEXP
 #endif /*  __GCJ_DLL__ */

-#else /* !( _WIN32 || __WIN32__ || WIN32) */
-
-#define JNIIMPORT
-#define JNIEXPORT
-#define JNICALL
-#define _CLASSPATH_JNIIMPEXP
-
-#endif /* !( _WIN32 || __WIN32__ || WIN32) */
-
-
 #endif /* __GCJ_JNI_MD_H__ */


-- 
           Summary: support for GCC4.0's fvisibility option in JNIEXPORT
                    macro
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: java
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jan dot nijtmans at gmail dot com
GCC target triplet: any target, except win32


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30999